The characteristic features of wetland soils are
the result of the activity of anaerobic
soil bacteria. Both obligate and
facultative anaerobic bacteria
exist in the saturated zones of many soils.
Legally, a soil is designated as a wetland soil when these characteristic
wetland features are present at or near the surface of the soil.

When soil is saturated and air is excluded from soil pores, anaerobic
bacteria dominate the soils microbic community. What do these microbes do
that aerobic
microbes cannot do? The main difference is that they use
substances other than oxygen molecules (O2) as their electron
acceptors. When most organisms respire, they transfer electrons from one
molecule to another, in a chain. Each time an electron is transferred, energy
is released that is used by the organism for its life processes. Ultimately,
that electron is transferred to an oxygen molecule. When O2 is no
longer available, the series of electron transfers stop. The aerobic organism
will die if O2 is not supplied.

Anaerobic organisms do not use O2 as their electron acceptors. There are a
number of substances that they will use as a substitute for O2, including
(among others) nitrate
(NO3-), ferric iron (Fe3+), manganese (Mn2+), sulfate
(SO42-) and carbon dioxide (CO2). (See related section: Biogeochemistry.)

Since Fe3+ is quite prevalent in most soils, anaerobic
bacteria add an
electron to it and thereby reduce it to Fe2+ (ferrous iron). Iron oxides
containing the Fe3+ form of iron are yellow or red in color and are insoluble
in water. Fe2+ forms are colorless and water soluble. Saturated soils are
therefore gray or white in color, while dryer soils are yellow or red. Areas of
the subsoil that are mottled with gray, yellow and red are areas with a
fluctuating water table (alternating between saturated and unsaturated
conditions). The presence of gray colors or mottles near the surface is an
indicator of a wetland soil. Black manganese nodules are also sometime present
in wetland soils, and serve as another indicator.

Anaerobes also reduce SO42- to H2S (hydrogen sulfide) producing the
characteristic rotten egg odor of certain wetlands (especially the Sulfaquents
and Sulfhemists of the salt marshes). Anaerobic bacteria are also less
efficient than aerobic
bacteria at decomposing organic matter. As a result,
wetland soils tend to have higher organic matter contents than unsaturated
soils. Often, the surface horizons are black or, indeed, entirely composed of
organic matter.

Wetlands are the habitat of
many unique and economically important plants and animals. However, equally
important to the well being of the environment are the processes that occur in
wetland soils.

There are three ways that wetland soils may filter contaminants from water:

Physical filtration occurs when moving water containing suspended
sediments floods a wetland. Often, the waters movement is slowed, perhaps
even to a stand still. Sediments then settle out of the water.

Biological filtration occurs with nitrates. In the case of
NO3-, anaerobic
bacteria use the nitrate
molecule as their final
electron acceptor, reducing it to N2 gas or various nitrogen oxides (also
gases). This process is especially important when agricultural fields are
adjacent to waterways.

Chemical filtration occurs because wetland soils are often high in
CEC (cation exchange capacity
). Positively charged particles are selectively
removed from the water.

In addition to water filtration, wetlands are important for flood control.
As mentioned above, wetland soils are usually high in organic matter, if they
are not largely composed of it. Soil organic matter absorbs many times its
weight in water, like a spongehence the spongy feel when one walks
through a wetland. Failure to maintain wetlands in a relatively natural state,
especially in flood plains, will result in greater losses when flooding occurs.

Further information on wetland soils can be found at:
http://www.ftw.nrcs.gov/soils_data.htm. For more information on federal wetland
laws, link to Federal Regulations
or visit the FEDLAW website at: http://www.legal.gsa.gov.